1. Field of the Invention
The invention relates to a knock control apparatus for an internal combustion engine that controls an operating state of the internal combustion engine based on the knock determination result, and a control method thereof.
2. Description of the Related Art
In an internal combustion engine (hereinafter also referred to as “engine”) of a vehicle or the like, ignition timings largely effect, for example, exhaust gas, fuel consumption, drivability (i.e., output torque of the engine), and so on. Therefore, ignition timings are controlled so as to be optimal for the operating state of the engine. When controlling ignition timings, an ECU (Electronic Control Unit), for example, calculates the optimal ignition timings based on the operating state of the engine (e.g. speed and load of the engine), and activates ignition plugs at the calculated ignition timings.
Further, in an engine of a vehicle, a KCS (Knock Control System) control that suppresses or prevents knocks of the engine is performed. In the KCS control, a filter for determining knocks (e.g. a band-pass filter) extracts the signals that fall in a knock frequency band (e.g. a frequency band with a center frequency of 7 kHz, 11 kHz, or 14 kHz) from among the signals output from the knock sensor provided in the cylinder block of the engine or other. Whether knocks are occurring is determined by comparing the peak value of the extracted knock sensor signals and a predetermined knock determination level. If it is determined that knocks are occurring, the ignition timings, in order to suppress or prevent the knocks, are retarded from the standard ignition timings so that the combustion speed of air-fuel mixture decreases and thus the maximum combustion pressure. If it is determined that knocks are not occurring, the ignition timing control is performed so that the ignition timing is gradually advanced to optimize the ignition timing.
The center frequency of the filtering frequency band of a knock detection filter used in KCS control as described above is set so as to match the actual frequency of knocks of an engine (hereinafter will be referred to as “knock frequency”). However, as the knock frequency changes due to age deterioration of the engine or the like, as shown in FIG. 6, a difference arises between the center frequency of the knock determination filter that has been initially set and the knock frequency, which reduces the accuracy in detecting knocks. As the technology for addressing this problem, a knock control apparatus is disclosed in Japanese Patent Publication Application No. JP-A-8-177697.
In the knock control disclosed in Japanese Patent Publication Application No. JP-A-8-177697, when knocks are actually detected, a difference between the frequency at which the knock sensor actually resonates and the set value of the resonant frequency of the knock sensor that is previously set and stored is detected. When the actual and set resonant frequencies are different, the set (stored) resonant frequency is corrected so as to eliminate the difference, and therefore good knock controllability is obtained.
In the knock control disclosed in Japanese Patent Publication Application No. JP-A-8-177697, when a deviation σ (or a peak value V) in the knock strength distribution of one of cylinders of the engine is smaller than a predetermined threshold, and the deviations σ (or peak values V) in the knock strength distributions of other cylinders are larger than the predetermine threshold, it is determined that the resonant frequency has deviated from the optimal frequency. Then, the filter characteristics of a resonant frequency switching circuit are switched, so that the center frequency is shifted to the optimal frequency. As such, in this control, the detecting and setting processes for obtaining the optimal frequency become very complicated. Further, since the threshold applied to the deviation of the resonant frequency from the optimal frequency depends on the engine speed and cylinder positions, there is a problem that matching of the threshold is difficult.